Petroleum products generally increase viscosity with decreasing temperature and vice versa, in other words, their viscosity is dependent on temperature. Automotive lubricating oils for example are required to have a less temperature dependent viscosity. In order to reduce the temperature dependence of viscosity, lubricating oils are mixed with viscosity modifiers that are specific polymers soluble in lubricating oil bases. Ethylene/(α-olefin copolymers are widely used as such viscosity modifiers, and the viscosity modifiers undergo various improvements to provide a further enhanced balance of lubricating oil performances.
The viscosity modifiers are usually used to prevent the lowering in lubricating oil viscosity at high temperatures. In recent years, however, there is a need for viscosity modifiers capable of suppressing the increase in lubricating oil viscosity at low temperatures (thereby providing excellent low-temperature properties). Further, excellent shear stability is required because lubricating oil compositions are generally used in applications where a shear force is applied. To improve the shear stability, low molecular weight polymers are usually used. However, viscosity modifiers having a low molecular weight have to be added in an increased amount so that the lubricating oils will have a desired viscosity, thereby causing economic disadvantages. Increasing the molecular weight of the viscosity modifiers and thereby reducing the amount of the modifiers will improve the economic efficiency of lubricating oils, but also deteriorate the shear stability.
In connection with the art in the above technical field, Patent Document 1 discloses a blend of ethylene/α-olefin copolymers differing in ethylene content.
This polymer blend is described to provide excellent low-temperature properties. However, the balance of performances other than low-temperature properties is insufficient.
Patent Document 2 describes ethylene/(α-olefin copolymers which have a nonuniform intramolecular composition, a narrow intermolecular composition distribution and a narrow molecular weight distribution.
In the production process disclosed in this patent document, the reaction is catalyzed by a single catalyst system in a tubular reactor in which the monomer concentrations are varied whereby the intramolecular composition is made nonuniform. The copolymer obtained therein is a so-called tapered copolymer in which the intramolecular composition in the polymer chain gradually changes and the intramolecular composition distribution is broad.
However, the above copolymer provides insufficient low-temperature properties. This is probably because the copolymer has an undesired segment because of the broad intramolecular composition distribution.
To solve this problem, Patent Document 3 discloses copolymers having a highly controlled intramolecular composition distribution.
As described in Production Examples in this patent document, the copolymers are produced by successive polymerization to highly control properties such as intramolecular composition distribution.
However, the successive polymerization is not industrially productive because materials having varied monomer proportions have to be supplied successively. Accordingly, simple production processes are desired.
Furthermore, Patent Document 4 discloses copolymers that are highly controlled in intramolecular composition distribution.
This patent document teaches relatively simple processes for producing the copolymers while highly controlling the intramolecular composition distribution.
However, the use of the copolymers as lubricating oil viscosity modifiers results in a very bad balance of lubricating oil performances.
Patent Document 1: U.S. Pat. No. 3,697,429
Patent Document 2: JP-A-S60-035009
Patent Document 3: JP-A-2003-48931
Patent Document 4: US 2006/0199896